The disclosure herein generally relates to implantable orthopedic prostheses and, more particularly, to an implantable knee prosthesis for measuring six different force components while the prosthesis is under load.
In the United States alone, over 200,000 knee replacements are performed each year. Degenerative arthritis, or the gradual degeneration of the knee joint, is the most common reason for these replacements. In this form or arthritis, cartilage and synovium surrounding the knee wear down so underlying bones grind directly on each other.
In knee arthroplasty, portions of the natural knee joint are replaced with prosthetic components. These components include a tibial component, a femoral component, and a patellar component. The femoral component generally includes a pair of spaced condyles that articulate with the tibial component. These condyles form a trochlear groove in which the articulating surface of the patellar component moves. The components are made of materials that exhibit a low coefficient of friction when they articulate against one another.
When the articulating ends of both the femur and tibia are replaced, the procedure is referred to as total knee replacement or TKR. Much effort has been devoted to performing TKR that restores normal, pain-free functions of the knee for the lifetime of the prosthetic components. Unfortunately, patients can experience problems with the prosthetic knee after a total knee replacement surgery. If a problem occurs, a patient may need a revision surgery wherein some or all of the prosthetic components are replaced.
Problems with a prosthetic knee can develop for a multitude of reasons. Many of these problems, though, could be eliminated or significantly diminished if scientists more thoroughly knew the dynamic forces that act on a prosthetic implant. As such, engineers and scientists devote much effort to understanding, measuring, and quantifying the forces on a prosthetic knee once it is implanted into a patient. If accurate information on these forces could be obtained, then designers could use this information to more accurately design a prosthetic knee.
Scientists have developed methods and apparatus to measure some of the forces on a prosthetic knee. U.S. Pat. No. 5,360,016 to Kovacevic and entitled xe2x80x9cForce Transducer for a Joint Prosthesisxe2x80x9d teaches an implantable knee prosthesis for measuring loads on the prosthesis during use. A transducer is disposed between two plates to measure axial forces on the prosthesis.
One major disadvantage with prior force measuring devices is the prosthesis can only measure forces in limited directions. The patent to Kovacevic, for example, measures axial loads on the implant. The prosthesis does not have the ability to measure three dimensional force components. In other words, forces on an implanted prosthesis actually occur along three different axes, the X-axis, the Y-axis, and the Z-axis. Measuring merely one or two of these components will not reveal a complete force distribution for the implanted prosthesis. In order to obtain this complete force distribution, forces in all three dimensions must be measured.
It therefore would be advantageous to provide implantable orthopedic prostheses that can measure three dimensional force components. Such prostheses would provide more complete measurements of the force distribution on the prosthesis.
The present invention is directed to implantable knee prostheses for in-vivo measuring force components along three different axes, the X-axis, the Y-axis, and the Z-axis. The prosthesis can measure six different load components along these axes while the prosthesis is under load. These components include the forces Fx, Fy, Fz, and the torques Tx, Ty, Tz.
The prosthesis generally comprises a tibial implant, a tibial shell, force detection instruments, and electronics. The tibial implant has a proximal end with a flat tray that has an elliptical shape. An elongated cylindrical stem extends distally from the tray. The stem is hollow and includes an opening at a distal end that leads into the hollow portion or cavity. A cap or plug is used to seal the cavity. This plug may be permanently connected to the stem, with welding for example, or removeably connected to the stem, with a press-fit or interference fit for example.
The tibial shell has a body with a cylindrical portion and a baseplate portion. A bore completely extends through the cylindrical portion from a proximal end to a distal end. The baseplate has a flat, elliptical shape that is similarly shaped to the tray portion of the tibial implant. The shell fits around the elongated stem portion of the tibial implant.
The force detection instruments are positioned inside the cavity of the cylindrical stem of the tibial implant immediately beneath or adjacent the tray. These instruments may be provided as strain gauges that are adapted to measure forces applied to the tray of the tibial implant. In the preferred embodiment, the force detection instruments are attached to an internal wall in the cavity of the stem. This portion of the stem acts as a spring element that deflects or moves when loads or forces are applied to the tray.
The electronics are positioned in the hollow portion or cavity of the stem of the tibial implant. These electronics are wired to the force detection instruments. Various electronic instruments may be provided and include, for example, an A-D converter, multiplexer, power receptor, radio transmitter, and on-board computer.
In order to assemble the components, the tibial implant and tibial shell can be connected together with an interference or tapered fit. Specifically, the stem of the tibial implant fits through the bore of the tibial shell until the tray of the implant and the baseplate of the shell are adjacent each other. Electronics and force detection instruments are then positioned inside the cavity of the stem of the tibial implant. A cap or plug then attached to the opening of the cavity to seal the electronics and instruments in the implant. The tibial implant and tibial shell can be assembled and calibrated outside of the patient. After the prosthesis is tested and validated, it can be implanted into the patient using surgical implantation techniques known in the art.
The prosthesis of the present invention measures loads on the surface of the tray portion of the tibial implant in a total knee arthroplasty (TKA) system. The force detection instruments are located on a resilient, measuring section of the cavity of the stem. This measuring section of the stem serves as a resilient, spring-like element. When loads or forces are placed on the tray, the measuring section deflects. This deflection is detected and measured with the force detection instruments. The electronics process these measurements and electronically relay the information to a computer.
One important advantage of the present invention is that a single prosthesis can measure six different load components while implanted. These load components occur along three different axes and include forces (Fx, Fy, Fz) and torques (Tx, Ty, Tz). The invention is not limited to a single axial measurement or a single torsional measurement. More comprehensive data can be measured and collected using the prosthesis of the present invention as compared to single measurement devices. This data provides a more complete account of the loads on a prosthesis while it is implanted in a patient.